Because Al—Cu—Mg (aluminum-copper-magnesium) alloy materials have comparatively high strength even among aluminum-alloy materials, they are used widely in structural elements in which high strength is required, such as, e.g., the outer skin of an aircraft. To increase corrosion resistance, a high-purity aluminum alloy, such as a 1000-series aluminum, is sometimes coated on the surface of this type of alloy material.
During flight of an aircraft, the difference between the plane's interior air pressure and the exterior air pressure changes frequently in accordance with changes in altitude. Owing to such fluctuations in the air-pressure differential, the load applied to the skin of the aircraft changes frequently. Because of this, there is a demand for the outer skin of an aircraft to possess not only excellent static strength, such as tensile strength, but also excellent fatigue characteristics.
To improve the fatigue characteristics of aluminum-alloy materials, alloys having various chemical compositions have been proposed in the past, e.g., in WO99/31287 (and its Japanese counterpart Japanese Laid-open Patent Publication 2002-508446) and in WO00/37702 (and its Japanese counterpart Japanese Laid-open Patent Publication 2002-533572).